Compound bows are a relatively recent innovation to the field of archery. However, due to their many advantages over conventional archery bows, the use of compound bows has increased and compound bows have been gaining rapidly in popularity.
In a conventional bow, the force required to draw the bow is determined by the bow stiffness and the draw force increases in a relatively uniform manner as the bow is drawn. The fact that the force required to draw the bow increases uniformly imposes limitations on the use of the bow which may limit its usage. By way of example, if the bow requires a draw force of 80 pounds, the archer must be able to, not only draw the bow, but to then hold the bow in a steady position during sighting and discharge of an arrow. While the archer may be able to draw a relatively heavy bow, he may not be able to hold the bow in a steady position during sighting and release of an arrow while maintaining the force on the bow necessary to keep it in its drawn condition.
In a compound bow, the draw force does not increase in a uniform manner as the bow is drawn. Through the use of eccentrically mounted pulleys positioned at the ends of the bow limbs, the effective length of the bow limbs is increased during draw of the bow through rotation of the eccentrically mounted pulleys. On rotation of the eccentrically mounted pulleys, the force required to maintain the bow in a drawn condition is decreased with the result that the force required to maintain the bow in a drawn condition during sighting and release of an arrow is less than the maximum force required in drawing the bow. In the case of a bow having a draw weight, for example, of 65 pounds, the maximum force required in drawing the bow is 65 pounds. However, near the end of the draw, there is a let off, e.g., of 10 to 15 pounds depending on the size and the eccentricity of the draw pulleys, with the result that the bow can then be held in a steady position during sighting and release of an arrow with a force of only 50-55 pounds. As the arrow is discharged, the draw pulleys undergo rotational movement which is opposite to the rotational movement that occurs during draw. With the reverse rotational movement of the draw pulleys, the force that is applied to the arrow by the bow is increased with the result that the arrow is discharged under a propulsive force that is higher than the force required to hold the bow during sighting and discharge of an arrow.
The energy which is stored in an archery bow during draw may be determined by integrating the area under the force-draw curve of the bow. In the case of a standard bow, the force-draw curve is a straight line in which, with the draw force plotted vertically and the draw distance plotted horizontally, the force increases uniformly as the bow is drawn with the draw force increasing to a maximum when the bow is fully drawn. However, in a compound bow, the force-draw curve is not a straight line but, rather, is a curve in which the draw force increases rapidly until a maximum is reached with the draw force then decreasing due to let off resulting from rotation of the eccentrically mounted draw pulleys. U.S. Pat. No. 3,486,495 to Allen contains an extensive description of a compound bow and the way in which a compound bow functions. As presented in the Allen patent, the stored energy represented by the area under the force-draw curve of a compound bow is greater than the amount of stored energy for an equivalent standard bow having the same draw length. Thus, another advantage of compound bows is the fact that they are more efficient in storing energy during draw of the bow. To store the same amount of energy with a standard bow, it would, thus, be necessary to use a bow with a greater draw length than that of an equivalent compound bow.
One problem area in the use of compound bows involves the accidental breakage of the bow by an overly enthusiastic archer. The mechanical advantage which is provided in a compound bow by the use of draw cables and draw pulleys permits the archer to continue the application of force to the bow after the bow has been drawn to its full draw length. The bow limbs of a compound bow are generally relatively short and stiff as compared with the limbs of a standard bow. The shorter and stiffer limbs are more susceptible to breakage and are more easily overstressed through the usage of multiple draw cables and draw pulleys which gives the archer a mechanical advantage in bending the bow limbs.
In view of the possiblity of limb breakage which is present in the use of compound bows, it would be desirable if a compound bow could be provided in which the mechanical advantage provided to the archer would be largely lost as soon as the bow was drawn to its full draw length. With such a compound bow, it would then be very difficult, if not impossible, for the archer to accidentally break a bow limb by overdrawing the bow.